How do I make multiple Blender objects into one?
Remove the objects you no longer want in your model, like the interior objects. Then select all of your objects that you want in a single model and hit Ctrl + J then click Join selected meshes. The multiple objects will now be one model you can export.

For further reducing the complexity of the model for better performance, you'll need to either remove more objects before joining them or you need to remove details from the objects you want to keep.